A man of God came from Judah by God's command to Bethel. Jeroboam was standing at the altar burning incense.
References for 1 Kings 13:1
By the LORD's word, the man of God cried out to the altar: "Altar! Altar! The LORD says this: Look! A son will be born to the house of David. His name will be Josiah. He will sacrifice on you, Altar, the very priests of the shrines who offer incense on you. They will burn human bones on you."
At that time the man of God gave a sign: "This is the sign that the LORD mentioned: ‘Look! The altar will be broken apart, and its ashes will spill out.'"
When the king heard the word of the man of God and how he cried out to the altar at Bethel, Jeroboam stretched his hand from the altar and said, "Seize him!" But the hand that Jeroboam stretched out against the man of God grew stiff. Jeroboam wasn't able to bend it back to himself.
The altar broke apart, and the ashes spilled out from the altar, just like the sign that the man of God gave by the LORD's word.
The king said to the man of God, "Plead before the LORD your God and pray for me so that I can bend my hand back again." So the man of God pleaded before the LORD, and the king's hand returned to normal and was like it used to be.